This invention relates to scanning radio receivers and, more particularly, to scanning radio receivers particularly useful on the frequencies assigned to the Public Safety Radio Services as well as other frequencies.
Scanning radio receivers are well known in the prior art and have found particular utility for the reception of radio signals on the frequencies assigned by the United States Federal Communications Commission to the Public Safety Radio Services. In the past, many such radio receivers used crystals as the tuning element to provide the necessary local oscillator signals and required the presence of one crystal for each frequency the receiver was capable of tuning. Examples of such receivers are those shown in U.S. Pat. Nos. 3,531,724 to G. H. Fathauer, 3,665,318 to S. J. Hoffman, et al., 3,714,585 to R. C. Koch, 3,725,788 to G. H. Fathauer, 3,794,925 to K. Imazeki, 3,801,914 to K. Imazeki, 3,821,651 to G. H. Fathauer, et al., 3,873,924 to G. H. Fathauer, 3,883,808 to J. E. Boone, 3,824,475 to P. W. Pflasterer, and 3,987,400 to G. H. Fathauer. Recently, scanning radio receivers using frequency synthesizing techniques have been provided which eliminated the need for a large number of crystals in radio receivers capable of being tuned to a large number of frequencies. Such receivers are shown in U.S. Pat. Nos. 3,937,972 to S. C. Snell, 3,961,261 to P. W. Pflasterer, 4,000,468 to J. R. Brown, et al., 4,027,251 to G. H. Fathauer, et al., 4,114,103 to P. W. Pflasterer, and 4,123,715 to G. H. Fathauer. There has also been provided by the prior art scanning radio receivers using frequency synthesizing techniques wherein the frequency synthesizing circuitry was controlled by the operation of a processing means such as a microprocessor. Exemplary radio receivers of this last mentioned type are disclosed in U.S. Pat. Nos. 3,962,644 and 4,092,594 both to W. Baker as well as the presently pending United States application Ser. Nos. 847,497 of G. H. Fathauer, et al., 847,566 of G. H. Fathauer, 000,905 of W. L. Williamson, et al., and 001,013 of A. Khan, et al.
The Federal Communications Commission specifies that frequency modulation be used on all the frequencies assigned to the Public Safety Radio Services. This specification, of course, has been a convenience in the past because it meant that radio receivers designed to receive signals at such frequencies need only include circuitry for responding to signals including one type of modulation, i.e., frequency modulation. However, it has become desirable to provide scanning radio receivers which have the capability of receiving signals at the frequencies assigned by the Federal Communications Commission for other services such as the aircraft communications service as well as the Public Safety Radio Service frequencies. The Federal Communications Commission has specified that amplitude modulation be used on the frequencies assigned for aircraft communication. Scanning radio receivers for use on the aircraft communication frequencies have previously been known, but those receivers were often capable of receiving only the aircraft communication frequencies and thus again only needed the facility for responding to signals of one modulation type, i.e., amplitude modulation. U.S. Pat. No. 3,725,788 to G. H. Fathauer discloses a radio receiver capable of scanning two Public Safety Radio Service frequencies and also of receiving conventional amplitude modulated broadcast signals. Even that receiver of now seemingly rather limited capabilities included completely separate radio frequency amplifier and mixer stages, local oscillator stages, intermediate frequency amplifier and filter stages, and demodulator stages for use with signals having the two different types of modulation. Further, it needed no circuitry to provide a scanning operation in response to amplitude modulated signals.
The problems involved in producing a single scanning receiver capable of receiving both the public safety and aircraft communication frequencies have not been clearly recognized and solutions have not been evident. A receiver including circuitry for demodulating both amplitude modulated and frequency modulated signals must perform under the operating conditions imposed by the environment of a scanning radio receiver.
To elaborate somewhat, in its scanning mode of operation a scanning radio receiver is successively tuned to different frequencies previously selected by the user until it arrives at a frequency at which a signal above a minimum signal strength is received. During the time it is tuned to any individual frequency, it must ascertain whether a signal of sufficient strength is present at that frequency, and this is usually done by evaluating some characteristic (such as high frequency noise content) of the demodulated signal. The scanning process is typically performed at the rate of as many as ten or more frequencies per second. Thus, if aircraft communication and public safety frequencies are to be scanned successively and intermixed in the order of scanning, it is necessary that some mechanism be provided for altering the demodulation operation of the receiver between frequency and amplitude demodulation depending on the form of modulation used at the frequency to which the receiver is tuned at any instant.
It is also desirable, or course, in the design of a scanning radio receiver to minimize the cost of the receiver to the extent possible while maintaining the performance level of the receiver. However, it has not been recognized that in a scanning radio receiver for receipt of both amplitude modulated and frequency modulated signals it is desirable to avoid the use of separate and independent circuitry for performing the necessary reception functions for amplitude and frequency modulated signals. An essential portion of a scanning radio receiver is the provision of a squelch or similar circuit to provide an output signal indicative of the presence or absence of a signal above a minimum signal level at the frequency to which the receiver is tuned. It is desirable to avoid separate and independent circuitry for providing such a signal with amplitude and frequency modulated signals. Further in a scanning radio receiver operable in a search mode wherein the receiver is successively tuned to each of the assigned frequencies within a predetermined frequency range until a signal above a minimum signal strength is found, it is generally necessary to include circuitry for preventing the receiver from erroneously responding to a received signal when it is not correctly tuned to that signal. (This problem and a solution thereto forms a part of application Ser. No. 847,566 referred to above.) However, it is also desirable to avoid the use of separate and independent such circuitry with amplitude and frequency modulated signals.